Field of the Invention
The present invention relates to an expandable composite resin bead containing an organic physical blowing agent and a base resin composed of a composite resin obtained by polymerizing a styrene monomer in a polypropylene resin.
Description of the Related Art
A foamed resin molded article formed from expanded beads obtained through in-mold molding and mutual fusion-bonding of the expanded beads is widely used in applications such as for wrapping materials, building materials, impact absorbing materials, and the like, by utilizing excellent properties of the foamed resin molded article including buffering property, lightweightness, thermal insulation property, and the like. As the foamed resin molded article, those whose base resin materials include a polyolefin resin such as polypropylene and polyethylene or a polystyrene resin such as polystyrene are used. An expanded bead is obtained by foaming and expanding an expandable resin bead containing a blowing agent, and an expandable resin bead containing a desired base resin is used.
When compared to a foamed polystyrene resin molded article formed from the expanded beads, a foamed polyolefin resin molded article formed from the expanded beads has excellent impact resistance, toughness, and post-compression restorability, and is therefore used as packaging materials and wrapping materials for precision components and heavy weight products etc. Furthermore, the foamed polyolefin resin molded article also has excellent heat resistance and oil resistance, and is therefore also used as automobile components such as impact absorbers, bumpers, and floor spacers. However, although the foamed polyolefin resin molded article has excellent characteristics as described above, the foamed polyolefin resin molded article also has shortcomings including having low rigidity and requiring equipment such as special metal molds since the steam temperature upon in-mold molding is high, when compared to the foamed polystyrene resin molded article.
On the other hand, when compared to the foamed polyolefin resin molded article, the foamed polystyrene resin molded article is generally moderate in price, has low molding vapor pressure upon in-mold molding, and has fine processability, and is therefore widely used in the market. Furthermore, the foamed polystyrene resin molded article has excellent rigidity, and enables a high expansion ratio depending on the application. Thus, when compared to the foamed polyolefin resin molded article, the foamed polystyrene resin molded article is also advantageous in terms of lightweightness.
However, when compared to the foamed polyolefin resin molded article, the foamed polystyrene resin molded article also has shortcomings such as having inferior toughness and post-compression restorability.
In order to resolve shortcomings of the both types of molded articles, for example, the following techniques have been developed. More specifically, an expanded bead containing a composite resin obtained by impregnating a polypropylene resin bead with a vinyl monomer and polymerizing the vinyl monomer has been developed (see Japanese Patent No. 4502234, Japanese Patent No. 5138254, Japanese Patent No. 5345329 and International Publication No. WO2007/099833). In Japanese Patent No. 4502234, an expanded bead containing 60 to 95% by weight of a polypropylene resin and 5 to 40% by weight of a polymer of a vinyl monomer is proposed. In Japanese Patent Nos. 5138254 and 5345329, an expanded bead containing 20 to 50% by weight of a polypropylene resin and 50 to 80% by weight of a polymer of a vinyl monomer is proposed. Besides, in International Publication No. WO2007/099833, an expanded bead containing 14.3 to 76.9% by weight of a polypropylene resin and 23.1 to 85.7% by weight of a polymer of a vinyl monomer is proposed.
The expanded bead described in Japanese Patent No. 4502234 still has, however, room for improvement in the rigidity of a molded article because a proportion of the polypropylene resin in the composite resin is high. Furthermore, the expanded beads of Japanese Patent Nos. 5138254 and 5345329 and International Publication No. WO2007/099833 still have room for improvement in the restorability because if the content of the polymer of the vinyl monomer is large, a molded article having excellent rigidity can be produced but the compression set of the molded article is large.
The present invention has been made in view of such a background, and provides an expandable composite resin bead having excellent expandability from which a composite resin expanded bead and a foamed resin molded article excellent in both rigidity and post-compression restorability can be obtained.